Antithetic Faults Research Articles

The Awakening of the Dormant Mount Vettore Fault (2016 Central Italy Earthquake, Mw 6.6): Paleoseismic Clues on Its Millennial Silences

AbstractThe Mount Vettore normal fault ruptured between August and October 2016, sourcing three earthquakes of Mw 6.2, 6.1, and 6.6. The first one caused the death of 299 people, while the entire sequence reached the highest macroseismic intensity levels in Italy since the catastrophic 1915 Fucino event (Mw 7.1). This fault was known to be one of the historically dormant faults of the Italian Apennines, and its sudden activation, not preceded by any foreshocks, has caught people and scientists off guards. We describe here the results from three new paleoseismic trenches opened across splays of the main antithetic fault that ruptured at surface on 30 October together with the 30‐km‐long Mount Vettore master fault. Data account for six surface faulting events since 9 ka, with a return time of 1.8 ± 0.3 kyr. The penultimate, probably stronger earthquake occurred in Late Roman times, perhaps in 443 AD, when also Rome suffered damage to its monumental buildings. Once again, paleoseismology turns out to be a powerful tool in seismic hazard assessment, especially for earthquakes that recur hundreds or thousands of years apart.

Pressure Transient Analysis of an Intersecting Rollover Faulted Crest Boundary in Niger Delta Oil Field

Pressure transient analysis has been extensively applied to detect anomalies in a reservoir system. These anomalies may be presented in the form of an intersection of the crestal and the antithetic fault associated with a growth fault. Interpretation of this fault can only be achieved through the use of pressure transient analysis. The objective of the research work is to analyze and test the faulted crest, depth of the anticline structure and examine the near well bore conditions in order to evaluate whether the well productivity is governed by wellbore effects (skin effects + well bore effect) or the reservoir at large. A case study of a well in the Niger delta is considered with a series of build up test carried out in two intervals of both upper and lower gauge readings. In this study, a computer aided design which uses a pressure derivative approach is used in this work to match the pressure derivative of an intersecting fault (angle) model to the field data, and the model assumes the characteristics of the reservoir. Based on the result of the interpreted data, simulation is done by using a non linear regression method (least square). The simulated data interpreted are achieved through the regression coefficient which provides a quantitative measure of the agreement between field data and the model. In conclusion, the best cases are taken from all the results and a nodal analysis is performed to diagnose the inflow performance of the well through the transient analysis in order to optimize the recovery of the oilfield.

Geomorphology, soils and palaeosols of the Chencha area (Gamo Gofa, south western Ethiopian Highlands)

The landscape of the Chencha Highland (south western Ethiopia) is characterized by flat plateau bordered by steep slopes affected by landslides and dissected by concave valleys and gullies. Coalescing alluvial fans are found along the eastern piedmont at the transition to the Lake Abaya shores. A major rift-plateau escarpment, with minor synthetic and antithetic faults, is located along the eastern slopes of the highlands. Soil erosion is a widespread process and the soil cover is usually thin and discontinuous. Immature Cambisols formed on colluvial deposits containing lithic tools, pottery fragments and charcoal represent the most recent phase of soil formation. They are related to the clearance of the original forest cover and the introduction of extensive agricultural and pastoral practices. On the slopes these soils locally unconformably overlie reddish, strongly weathered buried Nitisols and Luvisols that can be classified as Palaeoedlisols with formation of saprolite. Thicker and better preserved Palaeoeldisols can be found on the summit plateau, due to reduced runoff erosion. Different generations of clay illuviation indicate that they underwent polycyclic processes, recording soil processes probably older than Holocene. Along the slopes, gully erosion exhumed a palaeo-gully system infilled by colluviums of soils and buried soils containing Middle Stone Age artifacts suggesting that important slope degradational processes occurred also during the Late Pleistocene. Buried brownish soils in these infillings show moderate clay illuviation that indicates short-lived climatic amelioration and phases of slope stability.

Discrete-element modelling of hanging wall deformation along the D-1 fault system, Danish North Sea, with implications for gas chimney interpretations

This study uses 3D seismic mapping, NNCC attribute analysis and 2D SDEM modelling of soft-sediment deformation to analyse hanging wall deformation related to faulting across a listric fault detaching along the top of a major salt roller. The computational model experiments used the overall fault and salt-structure evolution as boundary conditions to predict the distribution of faults within the hanging wall. The modelling successfully reproduced geometries similar to the syn- and antithetic faults that are visible on seismics, but it also showed the presence of well-defined vertical deformation zones, both in the hanging wall and the foot-wall of the listric fault. In the model, salt movement controls the formation of the deformation zones. The similarity of NNCC-inferred vertical chimneys and the modelled deformation zones suggests that the subtle vertical chimneys identified in the 3D seismic data might actually be related to vertical to sub-vertical deformation zones caused by the presence and movement of the salt structure.

The Dinevar transtensional pull-apart basin, NW Zagros Mountains, Iran: a geological study and comparison to 2D finite element elastic models

The Dinevar transtensional pull-apart basin is a NW–SE trending basin located in the northwest part of the Zagros Mountains, Sanandaj–Sirjan tectonic zone, Iran. This basin was formed by oblique extension in a strike-slip fault system, which was controlled by the pre-existing Sahneh and the Mianrahan faults. In addition to a field study, we constructed a 2D finite element elastic model based on the major fault segments. According to the field study, various geological structures are formed around the pre-existing fault segments, including pencil structures with 13 < aspect ratio < 19, tight to open asymmetric folds, oblique–normal faults, synthetic and antithetic faults, normal and reverse fault drags, and negative and hybrid-type flower structure. FE-model results show that around the fault step is a zone of displacement gradient that indicates extension (transtension) within the overlapping zone. Consequently, subsidence is predicted over the releasing fault step. The modelled stress distributions within the fault step and around the fault segment tips are asymmetric. Elastic modelling shows that the mean normal stress within the step is tensional, which would impede plastic yield, while the region outside the step is characterised by compressive mean normal stress. The mean normal stress pattern is characterised by a regional minimum, localised inside the extensional step (transtension zone), and two local maximum and minimum couples, formed at the advancing and retreating sides, respectively, in the fault segment tips. The results show that two main subsidence regions (depocentres or sub-basins) form along and in the proximity of the fault segment tips. The imposed relative motion is entirely accommodated by creep on the faults, which causes plastic strain to accumulate outside the faults, mostly localised within the fault step. This localised strain between fault segments could initiate connecting faults or reactivate earlier faults, generally at high angles relative to the master fault segments as suggested by the stress and strain patterns. The rotation and magnitude variation in the stress and strain trajectories reflects heterogeneous deformation. In general, the Dinevar transtensional pull-apart basin is characterised by an elongate rhomboidal- to trapezoidal-shaped geometry. The modelled stress distribution and strain localisations within the basin are in general agreement with the locations of fault segments within the Dinevar transtensional pull-apart basin.

Hydrocarbon Reservoir Evaluation: a case study of Tymot field at southwestern offshore Niger Delta Oil Province, Nigeria

Abstract: Aim: This study presents the log analysis results of a log suite comprising gamma ray (GR), resistivity (LLD), neutron (PHIN), density (RHOB) logs and a 3D seismic interpretation of Tymot field located in the southwestern offshore of Niger delta. This study focuses essentially on reserves estimation of hydrocarbon bearing sands. Well data were used in the identification of reservoirs and determination of petrophysical parameters and hydrocarbon presence. Three horizons that corresponded to selected well tops were mapped after well-to-seismic tie. Structural depth maps were created from the mapped horizons. The structural style is dominated by widely spaced simple rollover anticline bounded by growth faults, and this includes down-to-basin faults, antithetic faults and synthetic faults. The petrophysical values – the porosity, net-to-gross, water saturation, hydrocarbon saturation that were calculated yielding an average porosity value of 0.23, water saturation of 0.32 and an average net-to-gross value of 0.62. Three horizons H1, H2 and H3 were mapped. The three horizons marked the tops of reservoir sands and provide the structures for hydrocarbon accumulation. Hydrocarbon in-place was estimated. The total hydrocarbon proven reserves for the mapped horizons H1, H2, and H3 were estimated to be 39.04MMBO of oil and 166.13BCF for sand E.

Tectonic patterns of shortening landforms in Mercury's northern smooth plains

Mercury's northern smooth plains are volcanically emplaced units characterized by ghost craters, volcanically buried impact basins, and thrust fault-related landforms. We analyzed the thrust fault-related landforms, traditionally categorized as lobate scarps and wrinkle ridges, within the northern plains in order to describe trends in how these landforms are organized and oriented and what style of deformation (either thin or thick-skinned) their map patterns represent. Our analysis also establishes geologic constraints for which global processes may have produced stresses contributing to these tectonic patterns. We mapped 4853 thrust fault-related landforms in the northern plains at a map scale of 1:1,000,000 using three MErcury Surface, Space, ENvironment, GEochemistry, and Ranging (MESSENGER) global monochrome mosaics. These landforms, described as curvi-linear asymmetric ridges, frequently occur in complex geometrical arrangements that are interpreted to share similar structural characteristics. We called these arrangements “compound landforms”. Like prior studies, we observed thrust faults to follow rims of buried craters. We also observed (1) sigmoidal rises bounded by fault-related landforms, (2) V-shaped rises composed of two landforms terminating at a single sharp point, (3) broad arcuate rises of nearly equal width, (4) parallel, evenly-spaced ridges, and (5) landforms showing alternation in direction of tectonic transport along strike. Respectively, we interpreted these landforms as transpressional uplifts, faults with sharply juxtaposing ramps, pop-up structures, fold and thrust belts, and antithetic fault intersections. By comparison with Earth analogues and patterns produced in numerical and physical models, our results suggest that deformation in the NSPs is thin-skinned. Orientation analysis showed that the northernmost landforms (90°–70°N) were predominantly oriented east–west while most of the landforms between 50° and 30°N were oriented north–south. Variations in orientation with latitude indicate that the growth of thrust fault-related landforms was influenced by sources of stress other than global contraction. If reorientation of the pole due to the formation of the Caloris basin impact did occur, the pattern of fault orientations indicates that geologic processes producing the pattern operated after reorientation.

Impact of structural setting on hydrocarbon trapping mechanism of onshore Niger Delta basin, Nigeria, using seismic attribute analysis

The evaluation of hydrocarbon trapping system in an onshore Niger Delta basin, Nigeria was studied using well log and seismic data. This is very important in estimation of production cost and assessment of viable reservoir units in petroleum industry. It is the main aim in reservoir characterization, in order to estimate the geometry and trapping mechanism of any particular field under investigation. These were achieved through the integration of 3D seismic and well log data that was acquired from an Onshore Niger Delta basin, Nigeria. From the well logs, three reservoir units was identified namely RS1, RS2, RS3 and these were mapped based on the curve signatures of the geophysical logs. These identified hydrocarbon bearing intervals were mapped on the seismic section using checkshot dataset for time to depth conversion. Four horizons and a total of thirteen fault traces were mapped within the region, with two characterized building structural faults of F2 and F3 features. Fault F4 and F7 are antithetic fault that trends north east to south west and the trapping mechanisms for the region are both a four way closure and fault assisted closure, as the trap is an up dip fault closure against the up thrown side of fault F8. Amplitude, variance and coherence attributes shows the faulting structure of hydrocarbon migration channels and path ways, through which hydrocarbon migration and accumulation occurs within the region. A significant level of contribution was achieved with the variance attributes showing a better resolution of an instant fault detection or identification compared to other attributes studied within the field of interest. The identified hydrocarbon bearing units lies within the same depth across the entire wells. This proved that the region has exhibits little or no tectonic deformation. The presence of structural features such as antithetic and synthetic is responsible for the hydrocarbon entrapment within the region. A detailed analysis of the study location revealed highly faulted region that exhibits a typical Niger Delta tectonic settings.

Architecture of tectonic sequences, depositional systems, and tectonic controls of the sedimentary fills of the rift‐related Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, China

Different sequence types were formed in various stages associated with tectonic variations and changes of the sediment supply in the rift‐related Wenchang Formation. Based on cores, wire logging, and seismic data, the sequence stratigraphy, depositional systems, and controls on the depositional systems of the Wenchang Formation in the Lufeng Depression were studied. The Wenchang Formation developed braided‐delta, lacustrine, fan‐delta, and nearshore subaqueous fan deposits and represents one first‐order sequence and three second‐order sequences, indicating three stages of tectonostratigraphic evolution. (a) The early syn‐rift sequence (SSQ1) developed fan‐delta and shallow lacustrine deposits. (b) The rift climax sequence (SSQ2) contains the early rift climax systems tract and the middle‐late rift climax systems tract. The early rift climax systems tract formed primarily braided‐delta and shallow lacustrine deposits, whereas the middle‐late rift climax systems tract developed mostly semi‐deep to deep lacustrine deposits. (c) The late syn‐rift sequence (SSQ3) developed braided‐delta and shallow lacustrine deposits. In the SSQ3 transgressive systems tract, retrogradational parasequence sets developed along the southern dip slope because of the weak sediment supply from the Huilu low uplift, whereas westward aggradational–progradational parasequence sets developed along the long axis because of the stronger sediment supply from the Lufeng eastern low uplift. Tectonic activity primarily controlled the early syn‐rift and rift climax deposition; syn‐depositional faults formed the transfer zones and slope break zones. Three types of transfer zones for sedimentary transport into the basin were distinguished, including synthetic approaching transfer zones, strike ramps, and collateral transfer zones. Three types of slope break zones were also recognised, including fault scarp zones controlling fan deltas and nearshore subaqueous fans, fault terrace zones controlling low‐stand fan deltas, and antithetic fault terrace dip‐slope zones controlling braided deltas.

IDENTIFYING FAULT ACTIVITY IN THE CENTRAL EVOIKOS GULF (GREECE)

Geological mapping on a 1:5.000 scale and a tectonic analysis in the wider Chalkida region of the Island of Evia and the adjacent Drossia area of Central Greece, have allowed the identification of a number of active and potentially active normal faults. These faults have been formed or reactivated during the Late Quaternary, since they affect Pleistocene brackish and terrestrial deposits. Some of the faults affect the contact of the limestone bedrock with the Quaternary formations, presenting characteristic polished surfaces. The faults, in places covered by the alluvial deposits of the Chalkida plain, are also detected by geophysical research. Among the identified faults, the most important are considered the Aghios Minas- Chalkida, the Avlida and the Lefkadi active faults. The first one extends from Drossia to the Chalkida area, crossing the sea straights, and has an ENE-WSW direction and a south dip. The other two, are parallel antithetic faults oriented WNW-ESE, and bound the South Evoikos Gulf on the Greek mainland and the Evia Island respectively. The mapping and evaluation of active faults in this region of moderate seismicity, with low topographic relief and consequent absence of morphotectonic features, is especially important from a seismic hazard point of view.

Surface ruptures following the 30 October 2016 Mw 6.5 Norcia earthquake, central Italy

ABSTRACTWe present a 1:25,000 scale map of the coseismic surface ruptures following the 30 October 2016 Mw 6.5 Norcia normal-faulting earthquake, central Italy. Detailed rupture mapping is based on almost 11,000 oblique photographs taken from helicopter flights, that has been verified and integrated with field data (>7000 measurements). Thanks to the common efforts of the Open EMERGEO Working Group (130 people, 25 research institutions and universities from Europe), we were able to document a complex surface faulting pattern with a dominant strike of N135°-160° (SW-dipping) and a subordinate strike of N320°-345° (NE-dipping) along about 28 km of the active Mt. Vettore–Mt. Bove fault system. Geometric and kinematic characteristics of the rupture were observed and recorded along closely spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, comprising a total surface rupture length of approximately 46 km when all ruptures were considered.

Seismicity During Continental Breakup in the Red Sea Rift of Northern Afar

AbstractContinental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late‐stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ∼64% of the seismic moment release and exhibits a swarm‐like behavior. In contrast, seismicity at the marginal graben is characterized by high‐frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low‐frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ∼12 km beneath Alu‐Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E‐W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long‐term extension.

Reservoir controlling differences between consequent faults and antithetic faults in slope area outside of source: A case study of the south-central Wenan slope of Jizhong Depression, Bohai Bay Basin, East China

Abstract The control effects of different occurrence faults on oil and gas accumulation and distribution in the outer slope area of oil and gas reservoirs were studied taking the south-central Wen'an slope of the Jizhong depression in the Bohai Bay Basin as an example. Based on 3D seismic data and the distribution of oil and water, the controlling differences between consequent fault and antithetic fault were analyzed and compared from the formation and evolution rule of faults and the formation mechanism of fault traps, including development positions of the consequent fault traps and antithetic fault traps, oil and gas distribution horizon adjusted by fault and formation period of fault traps. The differences between consequent faults and antithetic faults in controlling reservoirs have three main aspects: (1) Consequent fault traps and antithetic fault traps are in different positions, the consequent fault traps are at the segmented growing point in the hanging wall of “hard-linkage” faults, while the antithetic fault traps are developed in the position with the largest throw in the footwall because of tilting action; (2) The two kinds of faults result in different oil and gas distribution vertically, oil and gas adjusted by consequent faults is distributed in a single layer or multi-layers, while oil and gas adjusted by antithetic faults occur in single layers; (3) The two kinds of fault traps are formed in different periods, the consequent fault traps are formed at the time when the related faults enter the stage of “hard-linkage”, while the antithetic fault traps are formed at the beginning of the fault active period.

The controls of geomorphology and sediment supply on sequence stratigraphic architecture and sediment partitioning of the lacustrine rift basin in the Es3 of Liuzan area, Nanpu Sag, Bohai Bay Basin, China

ABSTRACTAn important hydrocarbon reservoir is hosted by the third member of the Shahejie Formation (Es3) in the Liuzan area, Nanpu Sag, Bohai Bay Basin, China. The Es3 can be divided into five third-order sequences (from base to top: Sq1, Sq2, Sq3, Sq4 and Sq5). Utilising well logs, cores and high-resolution 3-D seismic data, this study investigates the development characteristics and styles of sediment transport pathways in the steep-slope zone of the lacustrine rift basin. Two styles of sediment transport pathway are identified in the study area, including the faulted trough and fault slope-break zone. The faulted trough is divided into a single faulted trough, synthetic faulted trough and antithetic faulted trough. The fault slope-break zone is composed of synthetic fault slope-break zone and antithetic fault slope-break zone. The fan-delta plain and fan-delta front are recognised in the study area. The different styles of delivery conduits control depositional facies types and the spatial and temporal evolution of fan-delta depositional systems from Sq3 to Sq5. Based on paleo-geomorphological reconstruction, the dispersal pattern of the steep-slope zone system divided into a northern steep-slope system and an eastern steep-slope system is established. In Sq3, the northern steep-slope system develops large-scale mixed sandy–muddy fan-delta deposits derived from the NW. The eastern steep-slope system feeds the medium-scale gravel-rich fan-delta sediments. In Sq4, the large-scale mixed sandy–muddy fan-delta deposits develop from the NNE direction in the northern steep-slope system. The small-scale sand-rich fan-delta sediments are transported from the eastern steep-slope system. In Sq5, the large-scale mixed sand–mud fan-delta deposits are fed by both the northern steep-slope system and eastern steep-slope system. The strata stacking pattern exhibits a seesaw mode in the study area. Based on the analysis of the channel-belt thickness and the fan-scales, the paleogeomorphology of the drainage area in Sq3 and Sq4 and sediment supply in Sq5 alternatively control the sequence stratigraphic architecture and sediment partitioning in the sink area. The model proposed in this study may aid in the prediction of favourable reservoirs and good source–reservoir–seal development in lacustrine rift basin.

Sequence stratigraphic approach to hydrocarbon exploration: a case study of Chiadu field at eastern onshore Niger Delta Basin, Nigeria

This study integrated lithofacies, foraminifera and seismic data to develop a sequence stratigraphic approach to hydrocarbon exploration for Chiadu field. The application of lithostratigraphic approach led to correlation of diachronous lithofacies and wrongly defined an approximately accurate extent of the reservoirs. This study necessitated the use of sequence stratigraphic framework for the purpose of establishing chronostratigraphic concept and facies prediction. The genetic sequence model of Galloway was adopted for sequence stratigraphic interpretation of the field. The structural style is dominated by closely spaced simple rollover anticline bounded by growth faults, and this includes down-to-basin listric faults, collapsed crest structures, antithetic and synthetic faults. Result of foraminifera analysis showed that the analyzed interval is very rich in calcareous benthic but decreased in planktic and arenaceous benthic foraminifera. Lithofacies analysis shows that the continental lithofacies is made up of dominantly sand with thin bands of shales, while Agbada lithofacies is made up of alternating intervals of sandstones and shales to very thick shale unit in the lower section. The facies identified using log motifs and their stacking patterns include braided fluvial, crevasse splay, fluvial point bar, distributary/tidal channels, intertidal, subtidal and storm-dominated shelf facies. The presence of water depth indicator fossils confirmed the water depths of coastal deltaic, shallow inner neritic, inner neritic, middle neritic and outer neritic were based largely on the presence of environmentally restricted benthic foraminifera species. Integration of lithofacies and biofacies data suggests depositional environments ranging from coastal deltaic to outer neritic environment. Sequence stratigraphic analysis identified three complete sequences with accompanying system tracts over the interval 7922.3–14,856.34 ft. The transgressive system tracts (TSTs) within these sequences are dominated by marine shales and thin sands. Reservoir quality sands are found in highstand system tract and shelfal lowstand system tract, while the shales of TSTs and HSTs form potential source and seal units. The delineation and correlation of sequence stratigraphic surfaces enable us to build an approximate chronostratigraphic framework, which is essential for determining facies relationships. Reacquisition and/or processing should be done to improve seismic data quality for better imaging and interpretation/mapping, especially hydrocarbon prospect at deeper horizon.

Early Mesozoic intracontinental deformation in the eastern North China Block: Implication for an indentation model of North China to South China blocks

The Qinling‐Dabie‐Sulu Orogen was formed by the collision of the North and South China blocks during the Indosinian Period. The intracontinental deformation was subsequently developed during the Late Triassic to Early Jurassic in the northeastern, eastern, and southeastern parts of eastern North China Block with different structural patterns. In this paper, we present structural analysis of the Indosinian deformation in the western Shandong and southern Liaoning provinces and synthesized the previous studies of the Triassic‐Early Jurassic deformation in different areas of the eastern North China Block, from the northeastern, central to southeastern parts. Integrating the previous studies of petrology, geochronology, and geophysics, we suggested that (1) a series of top‐to‐the‐northwest thrust‐nappes were formed in the Sulu Orogen, and the southeastward antithetic faults developed in the South China Block. In the eastern North China Block, the Indosinian intracontinental deformation is intensely developed near the orogenic belt as thrust‐nappes and away from the belt developed open folds. (2) The present‐day structural lines in the northeastern, central, and southeastern parts of eastern North China Block show the S‐N‐, E‐W‐, and NE‐trending lines, respectively. It could be the structural response to the dynamics of an orocline or indentation induced by Triassic collision of the North and South China blocks; (3) the eastern margin of the North China Block subducted southeastward beneath the South China Block along the Wulian‐Qingdao‐Yantai Fault during the Indosinian Period.

Radon and carbon gas anomalies along the Watukosek Fault System and Lusi mud eruption, Indonesia

An extensive survey was carried out in the Sidoarjo district (East Java, Indonesia) to investigate the gas leaking properties along fractured zones coinciding with a strike-slip system, the Watukosek Fault System (WFS) in NE Java. This structure has been the focus of attention since the beginning of the spectacular Lusi mud eruption on the 29th May 2006. The sinistral strike-slip WFS originates from the Arjuno-Welirang volcanic complex, intersects the active Lusi eruption site displaying a system of antithetic faults, and extends towards the NE of Java where mud volcanic structures reside.In the Lusi region we completed a geochemical survey along three profiles combining measurements of a) 220Rn and 222Rn activity, b) CO2 and CH4 soil gas content, c) CO2 and CH4 fluxes, and d) gas analyses. The profiles are up to ∼1.2 km long and intersect perpendicularly areas with intense fracturing and surface deformation along the WFS. The purpose was to investigate the presence and origin of soil degassing activity in potentially active fault zones.Results show that the peripheral sectors of the profiles have high 220Rn activity and reduced CO2 and CH4 fluxes and concentrations. This suggests low fluids migration that could be affected by shallow circulation. In contrast, the segments of profiles intersecting the fractured zones have the highest 222Rn activity, CO2 and CH4 flux and gas concentration values.The relationship existing among the measured parameters suggests that the WFS acts as a preferential pathway for active rise of deep fluids. The presence of such advective processes is suggested by the relatively high rate of migration needed to obtain anomalies of short-lived 222Rn in the soil pores. Gas molecular and isotopic composition reveals that all sampled localities have a mixed hydrocarbon origin implying the presence of shallow microbial and deeper thermogenic hydrocarbons. CO2 isotopic values (δ13C-CO2 ranges between −9.48‰ and 4.12‰ V-PDB) indicate the presence of mantle derived CO2 and thermo-metamorphic CO2 suggesting that elevated temperatures have a key role in this active system. The samples collected from fractured and faulted zones reveal to have gas composition similar to that obtained from Lusi crater, indicating deep origin fluids.

Geodetic model of the 2016 Central Italy earthquake sequence inferred from InSAR and GPS data

AbstractWe investigate a large geodetic data set of interferometric synthetic aperture radar (InSAR) and GPS measurements to determine the source parameters for the three main shocks of the 2016 Central Italy earthquake sequence on 24 August and 26 and 30 October (Mw 6.1, 5.9, and 6.5, respectively). Our preferred model is consistent with the activation of four main coseismic asperities belonging to the SW dipping normal fault system associated with the Mount Gorzano‐Mount Vettore‐Mount Bove alignment. Additional slip, equivalent to a Mw ~ 6.1–6.2 earthquake, on a secondary (1) NE dipping antithetic fault and/or (2) on a WNW dipping low‐angle fault in the hanging wall of the main system is required to better reproduce the complex deformation pattern associated with the greatest seismic event (the Mw 6.5 earthquake). The recognition of ancillary faults involved in the sequence suggests a complex interaction in the activated crustal volume between the main normal faults and the secondary structures and a partitioning of strain release.

Development of branching brittle and ductile shear zones: A numerical study

AbstractContinental collision zones are usually associated with large‐scale strike‐slip shear zones. In most cases, these shear zones are complex and consist of multiple strands, varying in width, length, and total displacement. Here we present 2‐D numerical models to simulate the formation of such shear zones at different depth levels within the crust, under either brittle (frictional/plastic) or ductile conditions. Localization of shear zones is initiated by a material contrast (heterogeneity) of the material parameters. We systematically test the rate of strain weakening in brittle and in ductile regimes to understand its influence on the development of shear zone networks. Our simulations suggest that the development of antithetic faults in a brittle shear zone system is closely linked to a decrease in the angle of friction during deformation. In general, variation of the strain weakening also has a significant influence on ductile shear zones. Numerical results show that the geometry and thickness of the localized high strain zone are especially affected by weakening mechanisms during deformation. Furthermore, the interconnection and interaction of the shear strands lead to a more complex kinematic pattern, which lead to a local change in the maximum principal stress axis. These interaction of shear strands may explain the occurrence of shear‐related structures (e.g., folds) or differing characteristics of shear zones, such as the thickness of shear zones or the orientation of the faults to the stress field, which are consistent with field observations.

Imaging of subsurface faults using refraction migration with fault flooding

We propose a novel method for imaging shallow faults by migration of transmitted refraction arrivals. The assumption is that there is a significant velocity contrast across the fault boundary that is underlain by a refracting interface. This procedure, denoted as refraction migration with fault flooding, largely overcomes the difficulty in imaging shallow faults with seismic surveys. Numerical results successfully validate this method on three synthetic examples and two field-data sets. The first field-data set is next to the Gulf of Aqaba and the second example is from a seismic profile recorded in Arizona. The faults detected by refraction migration in the Gulf of Aqaba data were in agreement with those indicated in a P-velocity tomogram. However, a new fault is detected at the end of the migration image that is not clearly seen in the traveltime tomogram. This result is similar to that for the Arizona data where the refraction image showed faults consistent with those seen in the P-velocity tomogram, except that it also detected an antithetic fault at the end of the line. This fault cannot be clearly seen in the traveltime tomogram due to the limited ray coverage.